The invention relates to a combined process for the preparation of polycarbonate by the phase boundary process and electrolysis of sodium chloride-containing process wastewater.
The preparation of polycarbonates by the phase boundary process is generally known. It is usually effected in a continuous process, by preparation of phosgene and subsequent reaction of bisphenols and phosgene in the presence of alkali and a catalyst, preferably a nitrogen catalyst, chain terminators and optionally branching agents in a mixture of aqueous alkaline phase and organic solvent phase, at the boundary.

The preparation of polycarbonates, for example by the phase boundary process, is described in principle in the literature, see Schnell, “Chemistry and Physics of Polycarbonates”, Polymer Reviews, Volume 9, Interscience Publishers, New York, London, Sydney 1964, pages 33-70; D. C. Prevorsek, B. T. Debona and Y. Kesten, Corporate Research Center, Allied Chemical Corporation, Morristown, N.J. 07960: “Synthesis of Poly(ester Carbonate) Copolymers” in Journal of Polymer Science, Polymer Chemistry Edition, Vol. 18, (1980)”; pages 75-90, D. Freitag, U. Grigo, P. R. Müller, N. Nouvertne’, BAYER AG, “Polycarbonates” in Encyclopedia of Polymer Science and Engineering, Volume 11, Second Edition, 1988, pages 651-692, and finally by Dres. U. Grigo, K. Kircher and P. R-Müller “Polycarbonate” in Becker/Braun, Kunststoff-Handbuch [Plastics Handbook], Volume 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester [Polycarbonates, polyacetals, polyesters, cellulose esters], Carl Hanser Verlag Munich, Vienna 1992, pages 118-145.
In the preparation of polycarbonates, the phase boundary process, also referred to as two-phase boundary process, has proved its worth for many years. The process permits the preparation of thermoplastic polycarbonates in a number of fields of use, such as, for example, for data media (CD, DVD), for optical applications or for medical applications.
Frequently, good thermal stability and a low degree of yellowing are described as important quality features for polycarbonate. Little attention has been paid to date to quality of the wastewater obtained in the preparation of polycarbonates. In particular, the pollution of the wastewater with residual organic substances, such as, for example, residual phenols, is of importance for a possible further treatment of the wastewater, for example by a wastewater treatment plant or by ozonolysis for oxidation of the residual organic substances. In this context, there have been a number of applications which, however, predominantly describe methods for subsequent wastewater treatment with the aim of reducing the pollution by phenolic components—cf. for example: JP 08 245 780 A (Idemitsu); DE 19 510 063 A1 (Bayer); JP 03 292 340 A (Teijin); JP 03 292 341 A (Teijin); JP 02 147 628 A (Teijin).
The pollution of the wastewater with residual organic substances, such as, for example bisphenols or, for example, phenols used as chain terminators can be kept low if a large excess of phosgene is employed. This is, however, undesired for economic reasons.
In the preparation of polycarbonates with a reduced excess of phosgene, there is the danger that not all bisphenol or all monophenol will react and the wastewater will be polluted. Furthermore, there is the danger that the phase separation and the scrubbing are complicated in that the surface-active phenolic OH groups remain in the polymer. As a result, it is possible that not all water-soluble impurities will be extracted from the organic phase. This in turn can adversely affect the product quality.
It remains to state that the preparation of polycarbonate of high quality by a continuous two-phase boundary process with simultaneously low pollution of the wastewater is possible either only with a large excess of phosgene or with phase separation problems—associated with declines in the quality of the polycarbonate—or by subsequent treatment of the wastewater, as a result of which the cost-efficiency of the process is reduced.
DE-A 42 27 372 of the applicant, in which the arrangement of the apparatus of the process according to the invention has already been described, is to be considered as the most obvious prior art. In contrast to the teaching according to the invention, however, DE-A 42 27 372 provides no teaching at all regarding quantity ratios and especially regarding circulation ratios in which the starting materials are combined, to say nothing of the fact that particularly low wastewater pollution with residual organic substances, such as phenols and bisphenols, can be achieved by means of specially established quantity and circulation ratios.
In these known processes, however, a higher residual content of bisphenols or phenols—also referred to below as residual phenol content—requires complicated purification operations in the wastewater of these processes which can pollute the environment and present the wastewater treatment plants with a greater wastewater problem.
Usually, the sodium chloride-containing solution has to be freed from solvents and organic residues and must then be disposed of.
However, it is also known that, according to EP 1 200 359 B1 (WO 2000/078682 A1) or U.S. Pat. No. 6,340,736, the purification of the sodium chloride-containing wastewaters can be effected by ozonolysis and is then suitable for use in sodium chloride electrolysis. A disadvantage of the ozonolysis is that this process is very expensive.
According to EP 541 114 A2, a sodium chloride-containing wastewater stream is evaporated down until complete removal of the water, and the remaining salt with the organic impurities is subjected to a thermal treatment, with the result that the organic constituents are decomposed. The use of infrared radiation is particularly preferred here. A disadvantage of the process is that the water has to be completely evaporated, so that the process cannot be carried out economically.
According to WO 03/070639 A1, the wastewater from a polycarbonate production is purified by extraction with methylene chloride and then fed to the sodium chloride electrolysis. By means of the process described, however, only at most 14% of the sodium chloride can be recovered from the wastewater of the polycarbonate production since, the water introduced into the electrolysis with the wastewater will shift the water balance of the sodium chloride electrolysis out of equilibrium if larger amounts are used. The precondition of this is that a wastewater having an NaCl content of 10% is used and a water transport of 4 mol of water per mole of sodium is effected by the ion exchanger membrane in the NaCl electrolysis.
The sodium chloride-containing solutions which are obtained in the polycarbonate production typically have a sodium chloride content of 6 to 10% by weight. Thus, the total sodium chloride present in the solutions can never be recovered. At a sodium chloride concentration of 10% by weight, only the use of about 13% of sodium chloride from the sodium chloride-containing solutions is possible in the standard sodium chloride electrolysis with a commercially available ion exchanger membrane which exhibits water transport of 3.5 mol of water per mole of sodium. Even with a concentration to a saturated sodium chloride solution of about 25% by weight, only a recycling proportion of 38% of the sodium chloride present in the sodium chloride-containing solution will be possible. Complete recycling of the sodium chloride-containing solution has not been disclosed. According to WO-A 01/38419, the sodium chloride-containing solution can be evaporated down by means of thermal processes, so that a highly concentrated sodium chloride solution can be fed to the electrolysis cell. However, evaporating down is energy-intensive and expensive.
Starting from the prior art described above, it is the object to provide a process which gives products in high purity and good yield, and at the same time represents a reduction of the environmental pollution or wastewater problem in the wastewater treatment plants by maximized recycling of sodium chloride and sodium chloride-containing process wastewater solutions which are obtained from polycarbonate production.
In particular, it should be taken into account in the recycling that the conversion of sodium chloride into chlorine and sodium hydroxide solution and optionally hydrogen has to be effected with the minimum use of energy and therefore also in a manner which spares resources.
It has now been found that, in the continuous preparation of polycarbonate by reaction of bisphenols and phosgene in an inert solvent or solvent mixture in the presence of base(s) and catalyst(s), improved recycling of sodium chloride from the sodium chloride-containing wastewater solutions obtained at the boundary can be achieved without complicated purification after a pH adjustment to a pH of less than or equal to 8 and after simple treatment with active carbon, by direct feeding to an electrochemical oxidation of the sodium chloride present to chlorine, sodium hydroxide and optionally hydrogen, it being possible for the chlorine to be recycled for the preparation of the phosgene.